Red maple floodplain forest (S2S3)
Red maple floodplain forests occur on low floodplains of minor rivers and along tributaries of major rivers. They are common in southern and central New Hampshire, but absent from the northern part of the state. River channels are typically 6–30+ m (20–100+ ft.) wide with average summer water depths of 0.6–0.9+ m (2–3+ ft.). Small to moderate-size watersheds are typical upstream of red maple dominated floodplains.
Close to the river, soils are usually somewhat poorly drained fine sandy loams and silt loams with very shallow or no organic horizons. At slightly higher elevations, soils may be to moderately well drained. Soil pHs average 5.1. Low floodplain soil surfaces are usually temporarily inundated during spring flood events. Reddish mottles occur at an average depth of 10 cm (4 in.) on the low floodplain to nearly 20 cm (8 in.) on the high floodplain. In contrast to most swamps, hummock-hollow microtopography is poorly developed or absent.
Characteristic vegetation: Red maple (Acer rubrum) dominates the canopy, with variable amounts of American elm (Ulmus americana), black cherry (Prunus serotina), and musclewood (Carpinus caroliniana ssp. virginiana). Tree canopy structure ranges from woodland (25–60% tree cover) to forest (>60% tree cover). Winterberry (Ilex verticillata), viburnums (Viburnum spp.), and other shrubs form a low to moderately dense understory. The herbs layer is typically well developed and often dominated by one or more ferns, such as sensitive fern (Onoclea sensibilis), royal fern (Osmunda regalis), northern lady fern (Athyrium filix-femina), and lesser amounts of marsh fern (Thelypteris palustris), cinnamon fern (Osmunda cinnamomea), and interrupted fern (O. claytoniana).
Variants: Three variants are described. Most of the species mentioned in the descriptions are not individually diagnostic of each variant, but each species assemblage as a whole is distinct. Most higher floodplain forests are set farther back from the river channel than the lower variants. In other locations, higher floodplains occur along natural levees and other areas adjacent to the main channel, and low floodplains are found farther from the main channel. At many sites, high to low floodplain forest variants form complex mosaics with other floodplain communities.
1. Low variant
This variant typically occurs at a low floodplain elevation, close to the river, giving it a wetter character. It has a fairly open tree canopy, high cover of graminoids and vines, and mottles occurring at a shallow depth in the soil. Plants indicative of wet conditions are abundant, including species such as American elm (Ulmus americana), silky dogwood (Cornus amomum), and spotted touch-me-not (Impatiens capensis). Other characteristic species include western black raspberry (Rubus occidentalis), common elderberry (Sambucus canadensis), speckled alder (Alnus incana ssp. rugosa), Virginia creeper (Parthenocissus quinquefolia), fox grape (Vitis labrusca), groundnut (Apios americana), poison ivy (Toxicodendron radicans), false nettle (Boehmeria cylindrica), sensitive fern (Onoclea sensibilis), white turtlehead (Chelone glabra), showy yellow wood sorrel (Oxalis stricta), white avens (Geum canadense), and rough bedstraw (Galium asprellum).
2. Medium variant
This variant is closely related both environmentally and floristically to the low variant. Both variants may be found adjacent to the channel at relatively low floodplain elevations, though this variant can also occur at slightly higher elevations and at greater distances from the channel. Red maple dominates the tree canopy. Black cherry (Prunus serotina) is generally more prevalent in the canopy than in the low variant. Other characteristic species include musclewood (Carpinus caroliniana ssp. virginiana), nannyberry (Viburnum lentago), witherod (Viburnum nudum var. cassinoides), and flat-topped white aster (Doellingeria umbellata).
3. High variant
This variant typically occurs 1-3 ft. higher on the floodplain than the other variants, and resembles mesic, mixed hardwood - conifer forests of the transitional or central hardwood region (e.g., hemlock - beech - oak - pine forest). American elm (Ulmus americana) and black cherry (Prunus serotina) are sparse or absent. Characteristic species include red maple (Acer rubrum), red oak (Quercus rubra), white pine (Pinus strobus), highbush blueberry (Vaccinium corymbosum), lowbush blueberry (Vaccinium angustifolium), wintergreen (Gaultheria procumbens), sweet pepperbush (Clethra alnifolia), winterberry (Ilex verticillata), witch hazel (Hamamelis virginiana), and black huckleberry (Gaylussacia baccata). Witherod (Viburnum nudum var. cassinoides) is also frequent, and ferns typical of drier conditions are present. Species such as poison ivy (Toxicodendron radicans), musclewood (Carpinus caroliniana ssp. virginiana), and silky dogwood (Cornus amomum) are sparse or absent.
Good examples occur along the Ashuelot River (Surry), Contoocook River (Antrim), Blackwater River (Salisbury), Beaver Brook (Pelham), Lamprey River (Epping), the Exeter River (Exeter), Bearcamp River (Ossipee), Saco River (Conway), Swift River (Albany), and Indian Stream (Pittsburg).
Red maple floodplain forests often occur as part of a temperate minor river floodplain system.
red maple floodplain forest (low variant) along the Lamprey River in Epping (photo by Ben Kimball)
red maple floodplain forest (medium variant) along the Lamprey River in Epping (photo by Ben Kimball)
Red maple floodplain forest along the Blackwater River in Salisbury (photo by Ben Kimball)
Red maple floodplain forest along the Exeter River in Exeter, NH (photo by Ben Kimball)
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